Protein dependence of digoxin determination on the Abbott-TDX

The protein dependence of digoxin quantitation on the Abbott TDX was evaluated. An inverse relationship was found to exist between protein concentration and digoxin levels in samples that were deproteinized with 50 g/L trichloroacetic acid prior to assay. This was especially evident in the protein range from 30-60 g/L where a 30% decrease in digoxin concentration was observed. Recovery studies using 125I-digoxin showed that this decrease could be totally accounted for by a decreased recovery of digoxin in the trichloroacetic acid supernatant. The serum-based calibrators used to establish the standard curve on the instrument showed similar recoveries to serum samples. The reduced recovery does not pose accuracy problems as long as the protein concentrations of the patient samples and calibrators are similar. The clinical and operational implications of this observation are discussed.